The influence of suction on the unconfined shear strength of a Compacted Residual Soil

This paper investigates the variation in the shear strength of a residual soil of gneiss using results from unconfined compression tests. The soil was prepared based on the standard Proctor compaction test. The specimens were statically compacted at three different regions of the compaction curve (dry side, wet side, and optimum point), in an attempt to generate three different structures from the same soil. A total of 54 unconfined compression tests were conducted. To enable evaluations at several suction levels, the specimens underwent either wetting or drying after compaction. The initial suctions were determined by the filter paper technique. The cohesion intercepts obtained from tests conducted on samples compacted at optimum moisture content and wet of optimum are similar and located above the intercept cohesion defined by specimens molded from the dry of optimum. The test results allowed defining three suction intervals; their influence on the shear strength presents different characteristics. The cohesion intercept obtained for the three molding conditions, initially shows a linear variation. After the air entry point, the intercept cohesion presented a non-linear behavior. For values of suction greater than 1000 kPa, the shear strength remains practically constant.